Non-Uniform Scaling during UV unwrap. What is this?
It's because your object has been scaled on some axes more than others in object mode.

For example:



It should be fine, but if you are having problems you could try applying the scale by pressing CtrlA> Scale in object mode. This will apply the object mode scale "offsets" to the mesh data, resetting the offsets to 1 without  changing the appearance of the model. Note that this could affect modifiers such as bevel. See Why are the longer edges being beveled more?